1. Field of the Invention
This invention relates to a sheet feeding apparatus for separating and feeding originals or recording sheets one by one in a facsimile apparatus, a copying apparatus, a printer or the like.
2. Related Background Art
As a separating system for separating and feeding sheets one by one in a sheet feeding apparatus, there is widely known a reversely rotatable roller type sheet feeding apparatus using a reversely rotatable roller to which drive is transmitted through a torque limiter or a device functionally similar to a torque limiter. As such reversely rotatable roller type sheet feeding apparatus, there has been proposed a device in which, as described in U.S. Pat. No. 4,368,881 or Japanese Patent Publication No. 2-41487, the pressure contact force between a reversely rotatable roller and a separating roller bearing against it is automatically adjusted in conformity with the strength or weakness of the limit value of a torque limiter (which is created by irregularity or the like in manufacture or use).
The reversely rotatable roller type separation shown in U.S. Pat. No. 4,368,881 will be described here with reference to FIG. 12 of the accompanying drawings. A reversely rotatable roller R rockable by an arm 101 is urged against a separating roller F by a spring 102, and a drive force is transmitted from the pivot shaft 103 of the arm 101 to the reversely rotatable roller R through gears 104 and 105.
In this construction, when the drive force is transmitted to the reversely rotatable roller R, a moment in a direction for urging the reversely rotatable roller R toward the separating roller F is created in the arm 101 by a drive transmitting force and further, a moment for urging the reversely rotatable roller R against the separating roller F is created by a frictional force the reversely rotatable roller R receives directly or indirectly through a sheet from the separating roller F.
Accordingly, in such a construction, both of the drive transmitting force and the frictional force create a force which urges the reversely rotatable roller R against the separating roller F.
Describing the reversely rotatable roller type separation shown in Japanese Patent Publication No. 2-41487 with reference to FIG. 13 of the accompanying drawings, a reversely rotatable roller R has its shaft 106 slidably engaged with a groove 107a in a side plate 107 and is vertically movably provided, and is urged against a separating roller F by gravity. A drive transmitting force is transmitted to the reversely rotatable roller R by a belt 108.
In this construction, when a drive force is transmitted from the belt 108 to the reversely rotatable roller R, a force which urges the reversely rotatable roller R toward the separating roller F is created by the drive transmitting force. Also, a force which spaces the reversely rotatable roller R apart from the separating roller F is created by a frictional force the reversely rotatable roller R receives directly or indirectly through a sheet from the separating roller F. Also, the drive transmitting force creates a force which urges the reversely rotatable roller R against the separating roller F and conversely, the frictional force creates a force which spaces the reversely rotatable roller R apart from the separating roller F.
In the construction shown in the aforementioned U.S. Pat. No. 4,368,881, however, both of the drive transmitting force to the reversely rotatable roller R and the frictional force the reversely rotatable roller R receives from the separating roller F act to urge the reversely rotatable roller R against the separating roller F and therefore, the pressure contact force between the reversely rotatable roller R and the separating roller F sometimes becomes too strong, and if the pressure contact force is too strong, the limit value of the torque limiter will have to be set high in conformity therewith. However, if the limit value of the torque limiter is set high, a sheet conveying portion disposed at the downstream side will have to draw out a sheet against this strong pressure contact force and therefore, the conveyance speed will become irregular or a drive motor will cause a loss of synchronism, and this will lead to the problem that poor reading of originals and poor sheet feed are caused. Yet, if the conveying force of the conveying portion is set to a great level, it will give rise to problems such as the bulkiness of the motor and an increase in a driving current.
Also, in the construction shown in Japanese Patent Publication No. 2-41478, the frictional force the reversely rotatable roller receives from the separating roller acts to space the reversely rotatable roller apart from the separating roller and correspondingly thereto, it becomes difficult for bad conveyance by an increase in the pressure contact force to occur, but conversely, the reversely rotatable roller becomes liable to separate from the separating roller and separation cannot be sufficiently accomplished and double feed becomes liable to occur. So, it becomes necessary to provide a spring or the like of a strong biasing force discretely to urge the reversely rotatable roller against the separating roller, and in such case, unless the setting or the like of the spring force is accurately done, the force with which the reversely rotatable roller is urged against the separating roller will become too strong, and this will give rise to the same problem as in the above-mentioned U.S. Pat. No. 4,368,881.